Numerous tasks, for example, cutting sheet metal or abrading a surface, may be accomplished through the use of a stream of pressurized fluid, typically water, which is generated by high pressure, positive displacement pumps. (The term "high pressure" used throughout this application refers to any pressure above 10,000 pounds per square inch (psi), including up to and beyond 60,000 psi). Such pumps pressurize a fluid by having a reciprocating plunger that draws the fluid from an inlet area into a pressurization chamber during an intake stroke, and acts against the fluid during a pumping stroke, thereby forcing pressurized fluid to pass from the pressurization chamber to an outlet chamber where it is collected to be used by an operator via whatever tool has been attached to the pump for a particular task.
During the normal course of operation, the required flow rate will vary from the maximum the pump can supply to zero, for example, when the operator turns the tool off. In this situation, where the pressurized fluid is not being used, the pressure in the outlet chamber will build up beyond an acceptable level unless some form of pressure control is incorporated into the pump. If no pressure control is provided, the buildup of high pressure will result in damage and stress to the parts of the pump and undesirable surges of pressure will occur when the operator again turns the tool on.
One method of pressure control which is currently used is to incorporate a relief valve into the pump. When the pressure in the outlet chamber rises above a preset limit as a result of pressurizing more water than is demanded by the end user, the relief valve opens to vent the excess pressurized fluid. A relief valve may be direct acting, meaning that pressurized fluid acts directly to open a poppet that is being held in a closed position by a control force, such as a spring. Currently available direct acting relief valves for high pressure pumps typically last approximately 100 hours, their useful life being limited by severe erosion, which ultimately destroys the sealing ability of the valve. Various attempts have been made to engineer materials that will withstand the destructive effects of normal operating conditions; however, as the operating pressures of high pressure pumps have risen, these materials have failed to increase the useful life of the valves.